1. Field of the Invention
The present invention relates to an inverted telephoto type wide angle lens system having a relatively high aperture ratio of greater than 1/2.8, a relatively wide field angle reaching 75 degrees and a back focal length greater than the focal length of the lens system.
2. Description of the Prior Art
Retrofocus lens systems have been suggested in the prior art with a negative front lens group to obtain a sufficiently long back focal length. For example, a negative front lens group formed from a combination of a pair of negative single lenses and a positive single lens is suggested in U.S. Pat. No. 3,862,794.
The prior art has strived to provide an increased field angle while still maintaining a light weight compact lens structure. In attempts to achieve this compactness, it has been recommended to locate a positive single lens, preferably in the form of a positive meniscus lens convex to the object side, at the front of the lens system and to arrange a pair of negative lenses, preferably each in front of a negative meniscus lens convex to the object side, at the image side of the positive single lens. Reference is made to U.S. Pat. No. 3,973,832 to disclose such a lens system.
The competitive rigors of the camera industry has placed a constant demand on the lens designers to provide ever increasing optical performances while further compacting and making light weight lens systems. Accordingly, even though the prior art is crowded in attempts to achieve improved optical performance, there is still a demand to provide an improved inverted telephoto type wide angle lens system.